Jenn has washed ashore a small tropical island and it doesn’t take her long to realize she’s completely alone. She must spend her days not only surviving the elements, but must also fend off the malevolent force that comes out each night.
Jenn is the only survivor. She kills the shark-man Creature in battle and the island is left ablaze from their conflict. It is left open-ended as to whether Jenn will be rescued or otherwise escape from the island.
After their boat sinks during a storm, Jennifer Remming washes ashore a small, tropical island with her friend Brad, who dies from his wounds shortly after. While exploring the island, Jenn discovers the belongings and graves of a family that once lived there. She later buries Brad’s corpse in the sand, but discovers it savagely eaten and mutilated the following morning.
The next day, while retrieving her newly surfaced luggage, Jenn finds an ominous hole in the ocean’s floor. That night, Jenn fails to flag down an airplane using a flare gun and encounters a giant, two-legged sea monster. For the next four nights, Jenn manages to hide from the monster while also attempting to catch and kill it. As Jenn prepares for the fourth night, the corpse of another mutual friend, Zack, washes ashore mutilated and bisected. Jenn later uses his body as bait, seeing the monster clearly for the first time. For her fifth attempt, Jenn sleeps in a makeshift hammock in the trees to watch the monster arrive. When the hammock falls, Jenn stabs the creature with a sharpened stick and narrowly escapes.
The next day, Jenn is reunited with her boyfriend Lucas Griffin and friend Mia Reed after the two wash ashore in a lifeboat. Jenn warns Lucas and Mia about the creature, but they dismiss her claims. Jenn later discovers Lucas’ pocketknife bloodied. When Jenn makes a desperate attempt to flee without the two, Mia knocks her out with a boat paddle. Later, Jenn regains consciousness, finds herself tied up, and tries to defend her actions. Mia reveals Jenn’s past of fabricated lies, including one about being mugged before the group’s trip. When Mia leaves, Lucas hints to participating in Zack’s demise. Before Jenn can ask what happened to their friend, Mia is attacked by the monster. Lucas leaves Jenn tied up to fight it himself, but fails to save Mia, who is dragged into the water.
The next morning, Lucas and Jenn escape with the lifeboat, the latter discovering the inside covered in blood, presumably belonging to Zack. As the two head west, the sea monster attacks the lifeboat, eventually dragging Jenn away. When Jenn injures the monster, it releases her and drags Lucas to his death. Now alone, Jenn uses her journal to help any individual who may encounter the monster after washing upon the island. That night, Jenn sets some of the island on fire while battling the monster with a series of sharpened branches and bones from the graves of a family the monster killed. Both severely injured, the monster chases Jenn to the shoreline, but collapses from its wounds. Jenn decapitates the corpse and limps away from the blazing part of the island as the camera pans out.
A woman named Jenn (Kiersey Clemons) washes ashore on a deserted island. When she regains consciousness, she finds a man that she knows named Brad (Benedict Samuel) also washed up nearby. Brad has a large chunk of coral sticking out of his stomach. Jenn tries to treat Brad’s wound, but he dies from his injury. Before dying, Brad asks “did you see it?” Jenn is alone on the island. She pulls a flare gun off of Brad’s life vest. She investigates the small island and finds things left by a family previously stranded thereon. Jenn finds a small box of matches in their stuff.
After a storm at night, she finds a number of dead fish washed up on the shore. Jenn also finds a mutilated dead shark amongst them with large gashes along its side. As she wanders the island, Jenn comes across some graves of those stranded there before her. Jenn then buries Brad just along the shoreline. She uses one of the matches to build a fire to cook some of the fish she recovered from the beach.
The next morning, Jenn finds Brad’s grave open and his body missing. She realizes she must not be alone despite nobody else being on the small island. Jenn immediately makes herself a spear.
The following day, Jenn sees some of her luggage and other items floating just off the island. She swims out to retrieve them and sees a gigantic hole in the bottom of the ocean just off the shore.
That night, Jenn hears a plane fly overhead. She shoots off a flare to no avail. As the flare descends to the ocean, Jenn sees the silhouette of a creature (Andrew Crawford) walking out of the water and onto the sand. Jenn hears the creature making sounds as it walks nearby the foliage in which she hides. This is clearly the thing that stole Brad’s body. The creature does not find Jenn, and retreats to the ocean.
The next day, Jenn tries to swim off the island by floating on her luggage and life vest, but it is too unsteady. She finds a hollow log to sleep in. Jenn is awakened that night by the creature attacking the log, but it eventually loses interest and goes away.
When daylight returns, Jenn manages to kill a small shark with her spear. She suspends it from the top of a tall tree and hides in a nearby hole she dug in the sand. Jenn falls asleep. When she wakes up, she sees that the creature has pulled the shark off the tree (and presumably eaten it).
The corpse of another person Jenn knows floats ashore. She ties the corpse to the top of the trees and again lies in wait. This time, she stays awake long enough to watch the creature emerge from the ocean. It is enormous, easily reaching the corpse suspended towards the top of a tree. It is also clearly very strong. Although another plane flies overhead, Jenn cannot try to signal it with the creature on the shore. It eats the corpse and returns to the ocean
Jenn assembles a hammock near the top of some trees as far inland as she can. She hopes the hammock will provide safety from the creature. Late that night, the creature ventures inland. It manages to find and knock down the hammock. Jenn flees for her life, with the creature swimming just off the shore to keep up. It jumps back ashore and Jenn stabs it with her spear. The creature knocks Jenn aside and advances on her. However, a thunderclap scares the beast and it flees back into the ocean.
The following day, Jenn sees a life raft floating just off the island. On it are two more people she knows – her boyfriend Lucas (Emory Cohen) and Brad’s girlfriend Mia (Hanna Mangan Lawrence). They come ashore and embrace. Jenn urges them to flee the island on the raft as soon as possible. She tells them about the “monster.” Having almost died while trapped at sea and thinking Jenn is delusional, the other two refuse to leave. It is established that the boat they all were all on together sank.
Mia and Jenn discuss their experiences. Mia says dying on the raft would be misery, while Jenn maintains that staying on the island is even worse. When Lucas and Mia again refuse to leave, Jenn tries to depart on the raft by herself. The other two catch her in the act and knock Jenn unconscious.
Jenn regains consciousness tied to a tree at night. She immediately begins rubbing the rope binding her against the trunk to fray it and free herself. Mia talks about how Jenn is untrustworthy (due to incidents prior to the island) and that she doesn’t believe the creature exists. Lucas tries to talk to Jenn alone, but the couple begin to argue. It becomes apparent that the others on the boat were Lucas’ friends. He brings up that Jenn had a bad attitude on the boat and threatened to break up with him. Lucas starts yelling about how Jenn has no place to live, no money, and no life without him. He is indignant that given everything he does for her, she would abandon him on the island. Jenn doesn’t care about any of this and only demands to be let go.
The couple suddenly hears Mia screaming and the creature growling. Lucas runs towards the screams and sees the creature attacking Mia. He tries to fight it off with a torch, but it smashes him away. Jenn frees herself from her restraints and runs to the shore, just in time to see Mia dragged into the sea by the creature.
The next morning, Jenn convinces Lucas to flee on the raft with her. The couple lights a fire using the last match and cooks all the fish they have left. They load up the raft and set off.
The raft is attacked by the creature. Although they shoot their flares at it, the creature pierces the bottom of the raft and drags Jenn into the water. We get our first good look at it, revealing that the creature is a kind of shark-man (clawed arms and legs similar to a human, but a head and fins like a shark). As it drags Jenn towards its hole in the ocean floor, Jenn stabs it with a pocket knife and it releases her. Lucas jumps in to try and help but is attacked by the creature. As Jenn watches helplessly on, it grabs Lucas and drags him into its hole.
Jenn swims back to the island (where she finds the ruined raft beached on the sand) and saves the fire from going out. Knowing she has no other choice, Jenn begins preparing to fight the creature. She makes multiple spears and sets a bunch of fires. She then writes a letter detailing her time on the island in case she does not survive (noting that people have had a problem believing her for her entire life, but that they must believe her about this).
That night, the creature emerges once again. Jenn surrounds it with fire and stabs it several times. It eventually smashes her, knocking the wind out of her. She recovers just in time and continues beating it with branches and rocks or stabbing it with her knife. The creature lands a few blows, but nothing fatal. Finally, the creature collapses from its wounds and Jenn impales it on a spear.
Jenn decapitates the creature so she has proof of her tale. The camera pulls out to show 1/3 of the island on fire from the battle. Jenn stands victorious on the shore (but still lacking any means of escape).